test cases for version filters ...

- tests all combinations of inferred, specified, and inherited version filters

rewrite generator targets configuration ...

- as discusses in in https://github.com/dlang/dub/pull/1128 change the
  configuration algorithm to solve polluting sibling dependencies with
  compiler flags (e.g. versions), thus making it possible to better
  reuse cached builds

0. collect dependencies and shallow configuration
1. downwards inherit versions, debugVersions, and inheritable build options
2. define Have_dependency_xyz for all direct dependencies
3. upwards inherit dflags, versions, debugVersions, import paths,
    string import paths (and libs, lflags for static libs)
4. override string import files in dependencies

replace all usages of `exit` ...

- those skipped the ERR trap
- add die function to exit with line number and message

unify flags and error handling ...

- also add trap ERR handler to report exact error location

fix OSX test ...

- libcrypto has an additional dependency on libz
- use non-existent libraries to avoid any pkg-config differences

use DC instead of COMPILER ...

- so it's clear that it can be dmd/ldc2/gdc

mktemp on OSX requires a template

Fixed: --data=linker-files misses linker files specified in dub.json.

Change order of describe-data tests, list should run before dmd as it's easier to debug.